The present invention relates to the transmission and receipt of radio frequency (RF) energy in magnetic resonance spectroscopy and imaging systems. The present invention finds particular application in conjunction with dual frequency, quadrature radio frequency resonator coils for magnetic resonance imagers and will be described with particular reference thereto. It is to be appreciated, however, that the present invention will also find application in conjunction with tuning and impedance matching techniques for quadrature and non-quadrature as well as for single and multi-frequency RF coils.
Inductive coupling has previously been used in RF or resonator coils for magnetic resonance imaging systems as well as in other radio frequency and microwave applications. One use for inductive coupling has been to drive an RF resonator by coupling the resonator to the magnetic field flux generated by a separate conductive loop circuit. More specifically, in an excitation mode, the conductive loop circuit includes a primary of the inductive coupling which generates the magnetic field flux which is inductively coupled to a secondary circuit of the resonator which in turn, generates the RF excitation signal. In a receive mode, the radio frequency resonance signal detected by the resonator is inductively transferred to the "primary" loop circuit, which functions as the secondary in the receive mode. In this manner, only the primary circuit is connected directly to the transmitter and receiver. The resonator coil is floating. One advantage of inductive couplings is that the lack of physical interconnection with associated electrical devices renders it easy to retain quadrature symmetry. Another advantage is that spurious RF noises are reduced because a balanced feed to the RF resonator is automatically achieved with an inductive coupling.
In magnetic resonance imaging and spectroscopy, the input impedance when looking in to the driving port is matched to the characteristic impedance of the cables of the RF system, typically 50 Ohms. When the input and characteristic impedances are matched, the maximum possible current is provided in the secondary circuit, i.e. the resonator. Under this condition, the RF resonator efficiently excites and receives magnetic resonance signals from the nuclear spin system. The real or resistive component (R.sub.IN) and the imaginary or reactance component (X.sub.IN) of the input impedance (Z) can be expressed as follows: EQU R.sub.IN =R.sub.P +(2.pi.fM).sup.2 R.sub.S /(R.sub.S.sup.2 +X.sub.S.sup.2)(1) EQU X.sub.IN =X.sub.P -(2.pi.fM).sup.2 X.sub.S /(R.sub.S.sup.2 +X.sub.S.sup.2) (2),
where R.sub.P and R.sub.S are the resistance of the primary and secondary circuit, respectively, X.sub.P and X.sub.S are the reactance of the primary and secondary circuit, respectively, f is the frequency, and M represents the mutual inductance of the two inductively coupled circuits. In Equations (1) and (2), the second term on the right-hand side represents the coupled resistance and coupled reactance, respectively, from the secondary circuit. When the RF resonator is loaded, the resistance of the secondary includes both the resistance of the resonator circuit itself and the resistance from patient loading. To establish the maximum RF power transfer condition, the input resistance R.sub.IN should equal the characteristic resistance of the cable, e.g. 50 Ohms and the input reactance should be minimized or zeroed, at the coil resonance frequency f.sub.0, i.e.: EQU R.sub.P +(2.pi.f.sub.0 M).sup.2 R.sub.S /(R.sub.S.sup.2 +X.sub.S.sup.2)=50 .OMEGA. (3) EQU X.sub.P =(2.pi.f.sub.0 M).sup.2 X.sub.S /(R.sub.S.sup.2 +X.sub.S.sup.2) (4).
In the past, the critical coupling expressed in Equation (4) has been selected to couple the resonator and the primary driving circuit. To obtain the condition for the maximum possible secondary current, both the primary and secondary reactances X.sub.P and X.sub.S have been tuned to zero. That is, both the primary and secondary circuit have been on-resonance. The primary resistance R.sub.P has been typically much smaller than the second term of Equation (3) and was ignored. The second term of Equation (3) has been tuned to 50 Ohms. More specifically, the mutual inductance M has been tuned such that the real part of Equation (3) has been matched to the characteristic resistance of the cable 50 Ohms at the resonance frequency f.sub.0. Tuning the mutual inductance M has required changing the geometric shape of the primary circuit and the relative physical positioning of the primary and secondary circuits. These tuning changes have been made by mechanical adjustment. See for example, U.S. Pat. No. 4,638,253 of Jaskolski and Eash, and U.S. Pat. No. 4,939,465 of Biehl and Laukien.
Typically, the mechanical adjustments have involved rotational and dimensional changes in the primary coupling loop circuit. This required flexibility causes electrical instabilities in the RF cable due to the difficulties in maintaining the coupling circuits at the same position every time the coil matching and tuning are readjusted. Another disadvantage is that the primary/secondary coupling loop circuit has the same resonance frequency as the RF resonator. Fine tuning of the coupling circuit introduces an additional work load and tends to be very time consuming.
Yet another disadvantage is that the need to rotate the primary coupling circuit for tuning purposes causes the circuit to be disposed physically closer to the RF shield. The closer proximity to the RF shield introduces more RF currents in the shield near the primary coupling loop, disturbing the symmetry of the entire coil/shield system and introducing more circuit losses.
The present invention contemplates a new and improved inductive coupling technique which overcomes the above-referenced problems and others.